PIGMENT-CELLS. NAILS. HAIR. 251 



rate of growth in the nails of the hands is about 2-5ths of a line per week ; whilst 

 the nails of th^feet require four weeks for the same increase. Thus, the length 

 of the thumb-nail (including the portion hidden from sight) being 8 lines, the 

 period occupied in its growth would be twenty weeks ; whilst the nail of the great 

 toe, in like manner, being 9 lines in length, requires ninety-six weeks, or nearly 

 two years. It has been further remarked by M. Beau, that although the rate of 

 growth of the nails is not much affected by disease, the amount of nutriment they 

 receive is usually so much diminished, that the portion of nail then produced is 

 perceptibly thinner, and may be distinguished on the surface as a transverse 

 groove. The breadth of this groove indicates the duration of the disease, and its 

 depth marks the seriousness of the disturbance of the nutritive functions ; whilst 

 its distance from the root corresponds with the length of time that has elapsed 

 since recovery. 1 When a nail has been removed by violence, or has been thrown 

 off in consequence of the formation of pus beneath it, a complete regeneration 

 speedily takes place, provided that the matrix has received no serious injury. 

 The nail is continuous with the true Epidermis at every part, except at its free 

 projecting edge, where also the continuity is maintained in the foetus; so that it 

 may be regarded as nothing else than an extraordinary development of epidermic 

 structure, designed to answer certain special purposes of a purely mechanical 

 nature. 



245. The Hair, as originally consisting of Epidermic cells, may be properly 

 described here ; although, when fully formed, it departs widely (in Man at least) 

 from the cellular type. It has been imagined until recently, that the Hair, in 

 common with the other Epidermic tissues, is a mere product of secretion ; its 

 material, which is chiefly horny matter of the same composition with that of the 

 Epidermis and its appendages, being elaborated from the surface of the pulp at 

 its base. It is now known, however, to contain a distinctly organized structure ; 

 and to be formed by the conversion of a cellular mass at its root ( 246). Al- 

 though the Hairs of different animals vary considerably in the appearances they 

 present, we may generally distinguish in them two elementary parts ; a cortical 

 or investing substance, of a fibrous horny texture ; and a medullary or pith- 

 like substance, occupying the interior. The relative proportions in which these 

 present themselves, are subject to great variation; some hairs being almost 

 entirely composed of the medullary substance, and others almost as exclusively 

 of the cortical. The fullest development of both, however, is to be found in 

 the spiny hairs of the Hedgehog, and in the quills of the Porcupine, which are 

 but hairs on a magnified scale : their cortical substance forms a dense horny tube, 

 to which the firmness of the structure seems chiefly due ; whilst the medullary 

 substance is composed of an aggregation of very large cells, which seem not to 

 possess any fluid contents in the part of the hair that is completely formed, but 

 are occupied by air only. We shall see that in the Human hair, the predomi- 

 nant substance is that which corresponds to the " cortical" of that of the lower 

 animals. The diameter of the Hair is subject to great variation; ranging, on 

 the heads of different individuals (according to the observations of Mr. Erasmus 

 Wilson 3 ), from 1-1 500th to l-140th of an inch; and from l-1500th to l-230th, 

 even in the same individual. The average, however, seems to be about 1 -400th 

 of an inch, and is rather greater in the female than in the male. As a general 

 rule, flaxen hair is the finest, and black the coarsest ; and the most extensive 

 range is found in light brown hair. The hair of the beard and whiskers is con- 

 siderably coarser than that of the head ; the former having measured l-200th 

 of an inch, when the average of the latter was l-350th. When the surface of 

 the shaft of the Hair is carefully examined, it is seen to be covered with a layer 

 of flattened cells or scales, arranged in an imbricated manner (Fig. 43, c), their 



1 See Mr. Erasmus Wilson's "Healthy Skin," 3d edit. pp. 14-18. 2 Op. cit., p. 59. 



